Nickel metal hydride (NiMH) batteries are used in wireless electric tools and hybrid electric cars recently, which produces many spent NiMH batteries. Reuse and recycle of them is a must to eliminate possible pollutions, as well as to make sustainable production of them, as they use quite a lot of nickel, cobalt and rare earths. Here we report the separation of cobalt from Ni(OH)2 of spent NiMH batteries, which is required by a recycle process proposed by us, with a reduction and dissolution process in alkali solution. It shows that the alkali concentration plays a key role in the separation of cobalt from spent positive electrode materials. When the concentration of KOH solution increases from 1.0molL−1 to 11.0molL−1, the cobalt content in the solid materials continuously decreases from 5.09wt% to the minimum of 1.84wt% because of the reduction of the CoOOH (and/or Co3O4) formed during the charging of electrodes and dissolution of Co(OH)2 in the alkali solution. When the concentration goes further higher, however, it increases a little. The regenerated Ni(OH)2 remains the structure of β-Ni(OH)2. In addition, cobalt in the filtrate can be recovered by air oxidation and be reduced into Co(OH)2 by hydrazine, which can be again used as the conductive additive for Ni(OH)2 positive electrodes.